1. Field of the Invention
This invention relates to a fuel injection pump, and particularly to a fuel injection pump that has an improved fuel cutoff (spill rate) at the completion of the fuel injection.
2. Description of the Prior Art
In conventional fuel injection pumps provided on diesel engines, especially in-line type fuel injection pumps such as for example that of Japanese Patent Publication No. Sho 63-183264, the usual arrangement used to control the fuel injection amount and effective stroke consists of an inclined lead in the form of a spiral groove provided on the plunger, and a spill port formed in the control sleeve at a position corresponding to the position of the inclined lead.
This will now be explained with reference to FIGS. 3, 4 and 5. FIG. 3 is a cross-sectional side view of part of a conventional fuel injection pump 1 during the start of the fuel injection, and FIG. 4 is a cross-sectional side view of the pump at the completion of the fuel injection operation. The fuel injection pump 1 has a delivery valve/pump housing 2 and a plunger barrel 3 provided inside the pump housing 2, and has a fuel reservoir chamber 4 inside.
Extending from the pump housing 2 to the plunger barrel 3 is a plunger 5 that can rotate and reciprocate therein. Above the plunger 5 is a high-pressure fuel compression chamber and plunger chamber 6 that communicates with a delivery valve (not shown). The plunger 5 has a fuel suction and discharge port 7, a central fuel passage 8 that communicates the fuel suction and discharge port 7 with the plunger chamber 6, a vertical groove 9 that communicates with the fuel suction and discharge port 7, formed on the surface of the plunger, and an inclined control lead 10 that is also formed on the plunger surface, in communication with the vertical groove 9.
A control sleeve 11 is fitted over the plunger 5. A control rod (not shown) is used to move the control sleeve 11 vertically to change the relative positions of the control sleeve 11 and plunger 5 and thereby enable the prestroke to be adjusted. The control sleeve 11 is provided with a spill port 12 that passes radially therethrough. In addition, in the axial direction the location of the spill port 12 coincides with that of the inclined lead 10.
With the fuel injection pump 1 thus constituted, the descent of the plunger 5 causes fuel in the fuel reservoir chamber 4 to be drawn in via the fuel suction and discharge port 7, and the ascent of the plunger 5 causes the fuel suction and discharge port 7 to be closed by the lower end 11A of the control sleeve 11, starting pressurization of the fuel (FIG. 3). As shown by FIG. 5, which is an enlarged view of the fuel suction and discharge port 7, inclined lead 10 and spill port 12, as the fuel compression and injection timing are set by the lowermost point 7A of the fuel suction and discharge port 7, the lowermost point 10A of the inclined lead 10 must not be lower than the lowermost point 7A.
Again with reference to FIGS. 4 and 5, as the plunger 5 rises further, the upper edge 10B of the inclined lead 10 comes into alignment with the spill port 12 and the plunger chamber 6 and fuel reservoir chamber 4 are brought into communication via the central fuel passage 8, fuel suction and discharge port 7, vertical groove 9 and inclined lead 10, whereby fuel injection is terminated by a prescribed amount of fuel being spilled from the spill port 12 into the fuel reservoir chamber 4 (fuel spill timing). The fuel injection amount is controlled by a control rack (not shown) that rotates the plunger 5 about its axis to change the relative positional relationship between the inclined lead 10 and the spill port 12, thereby adjusting the effective fuel delivery stroke (FIG. 3). Also, the timing of the fuel injection can be advanced or retarded by using the vertical operation of the control sleeve 11 to adjust the prestroke.
With respect to a conventional fuel injection pump 1 employing these processes of fuel intake, compression, delivery and spill, it has been known that emission performance can be improved by improving the cutoff at the fuel injection termination by increasing the fuel spill amount per prescribed unit angle of pump rotation, that is, by increasing the spill rate.
In the case of Japanese Patent Publication No. Sho 63-183264, for example, the spill rate is increased by increasing the diameter of the spill port 12 formed in the control sleeve 11. However, increasing the diameter of the spill port 12 gives rise to various problems such as that it reduces the strength of the control sleeve 11 and thereby increases deformation, and, in addition, this arrangement limits the rate at which the area of the spill port 12 is opened by the lift of the plunger 5.
Moreover, using a higher pressure for the fuel injection tends to increase the effective stroke S, and with the shape of the inclined lead 10 used in the conventional arrangement, as shown particularly by FIG. 5, spill performance is degraded by the fact that the area of the port that is open at the termination of fuel injection at the high rack position (solid line) used to increase the effective stroke S is very small compared to the area at the low rack position (broken line).
That is, when the spill port 12 is near the end of the inclined lead 10, i.e. in the high rack position, the area of the spill port 12 that is open is limited to the portion (shown by cross-hatching) that is on the vertical groove 9 side (to the left, in the drawing) of the end point 10C on the long side of the inclined lead 10. While the inclined lead 10 can be further extended to ensure a sufficient port area in the high rack position, the start of the fuel injection operation to be mistimed if the lowermost point 10A is lower than the lowermost point 7A of the fuel suction and discharge port 7.